1. Field of the Invention
This invention relates to a gold alloy suitable for producing a colored gold alloy possessing a glossy black surface layer and used in ornaments of precious metal, a processed article of gold alloy, and a method for the production of the processed article.
2. Prior Art Statement
As ornaments of gold, processed articles of 18-carat gold, i.e. an alloy having a gold content of 75% by weight are widely available on the market. The 18-carat gold is characterized by possessing the stablest mechanical, physical, and chemical properties, excelling in workability, and permitting easy variation of hue. The processed articles of gold inherently have a gold hue and, because of this hue, are esteemed highly as ornaments. In recent years, processed articles of gold have become increasingly intricate in design and demand for such articles in a wider variety of hues has increased.
The hues in which the products of gold alloys meeting this demand are currently available include a light greenish yellow color of the Au-Ag alloy (Metal Data Book, page 186, Maruzen, 1984), a yellow color of the Au-Ag-Cu alloy (ibid.), a red color of the Au-Cu alloy (ibid.), and a light yellowish white color of the Au-Cu-Ni alloy (ibid.). Very recently, a purple color of the Au-Al alloy (Metal, Nov. issue, page 30, Agne's, 1984) and a yellowish green color of the Au-Cd alloy (Nonferrous Metals, II, page 231, compiled by Japan Metallurgical Society, 1986) have been developed.
With only five colors available (white, yellow, red, purple, and yellowish green), however, there are limits on the color variation of gold ornaments that can be obtained. Thus, the desirability of developing gold alloys of colors other than the colors mentioned above, particularly gold alloys of a black color forming a very fine contrast with the golden color, has been finding growing recognition.